Middle distillates, such as gas oil, Diesel oil or heating oil, which are obtained from crude oil by distillation have, depending on the source of the crude oil and depending on the type of processing in the refinery, different paraffin contents. The proportion of long-chain n-paraffins in particular determines the cold flow properties of such distillates. During cooling, the n-paraffins are separated in the form of platelet-like interlaced crystals which build up into a three-dimensional network (house of cards structure), where large amounts of still liquid distillate are locked up and immobilized. A decrease of flowability and an increase of the viscosity occurs parallel with the crystallization of the n-paraffins. The supply of middle distillates to the combustion means is made more difficult because of this. The precipitated paraffins plug filters ahead of the combustion means so that in extreme cases it is possible that the entire supply is stopped.
It has been known for a long time that the plugging of the filters at low temperatures can be overcome by the addition of so-called flow improvers. By means of the formation of nuclei, the additives cause the formation of many small paraffin crystals in place of a few large ones. At the same time they change their crystal modification, so that there is no formation of the above described platelets. The paraffin crystals formed in the presence of flow improvers are so small that they can pass through the filters, or they build up into a filter cake which is permeable to the still liquid portion of the middle distillate, so that operation free of disruption is assured even at low temperatures.
Middle distillate cuts are appearing in increasing amounts in the refineries, where the standard flow improvers do not have a sufficient effect or even fail completely. This applies particularly to so-called top draw oil, i.e. fractions with a high final boiling point (F.B.P.&gt;370.degree. C.). However, the boiling properties are not the criteria. It may occur in connection with two fractions with similar boiling point curves but dissimilar provenance of the basic crude oil, that the standard flow improver works well with one oil, but not with the other. In accordance with DIN 51 428, the effectiveness of the flow improver is indirectly expressed by measuring the cold filter plugging points (CFPP).
Ethylene copolymers, known per se, mainly copolymers of ethylene and unsaturated esters such as described in German Patent Disclosure DE-A-21 02 469 or European Patent Disclosure EP-A-84 148, are used as standard cold flow improvers.
However, the technology requires new flow improvers which also show good effectiveness in connection with the above described critical oils.
The use of polymers with linear, saturated side chains with at least 18 carbon atoms for reducing the flow point of paraffin-containing heating oil is known from German Patent Disclosure DE-A-16 45 785. Also recited are: "Reaction products of copolymers of acid anhydrides of unsaturated dicarboxylic acids and mono-olefins or other olefinic unsaturated compounds with an aliphatic amine containing a long hydrocarbon chain". In this case copolymers of mono-olefins are preferred.
In German Patent Disclosure DE-A-25 31 234 the addition of alternating copolymers containing maleic acid diamide or maleic imide structures are recommended as stabilizers in mineral oils, i.e. the carboxyl groups are completely reacted with amines into diamides or imides.
In accordance with U.S. Letters Pat. No. 3,506,625, reaction products of monoamines with maleic acid anhydride polymers to the corresponding imides are also described, where in case of use of less than one mol amine per mol unit of maleic acid anhydride still remaining carboxyl groups are changed to metal salts by neutralization. Alkylvinylether and monovinylhydrocarbons are preferably used for the copolymerization with maleic acid anhydrides.
French Letters Patent 2.592.658 describes mixtures of an ethylene polymer and a reaction product of a primary amine with a copolymer of, for example, acrylic acid lkylesters, diisobutene and maleic acid anhydride and their use as an additive to middle distillates.
Middle distillates are described in European Patent Disclosure EP-A-360 419, which contain polymers of vinylethers with hydrocarbon radicals of 1 to 17 carbon atoms. Alkylacrylates or -methacrylates, among others, are disclosed as co-monomers. However, the examples only describe polymers of alkylvinylethers with up to four carbon atoms in the side chain. These C.sub.1 - to C.sub.4 -vinylethers are copolymerized with derivatives of maleic or fumaric acid. No examples of copolymers with derivatives of acrylic acid are provided. The claimed additives can be used in conjunction with other flow improvers.
The use of polymers with at least one amide group from a secondary amine and a carboxyl group as an additive to middle distillates is known from European Patent Disclosure EP-A-283 293. The polymers can be obtained, for example, by copolymerization of unsaturated esters with maleic acid anhydride and subsequent reaction with the secondary amine. Among others, dialkylfumarate and vinylacetate are disclosed as unsaturated ester monomers.
However, these polymers leave a lot to be desired in regard to their effectiveness as cold flow improvers for middle distillates.
For these reasons the problem arose of finding additives to middle distillates with improved efficiency as cold flow improvers.